Plasma arc torches are commonly used for high speed, high precision cutting of metals. These torches use a transferred arc mode of operation wherein the torch includes an electrode which supports an arc extending from the electrode to a workpiece. Generally, a gas is energized by the arc to produce a plasma. The produced plasma arc is then directed by the plasma arc torch through a nozzle and toward the workpiece to be cut. The characteristics of the cut produced in the workpiece by the plasma arc are dependent upon different factors including torch operational parameters, the configuration of the nozzle, and the characteristics of the workpiece. Further, it is well known in the art that improved torch operation is obtained by injecting a flow of water through the nozzle to surround the plasma arc, thereby constricting the plasma and increasing the cutting ability of the torch. The water flow is also helpful to cool the nozzle, which increases its operational life. Water injection torches are also advantageous in cutting 0.5 inch and thicker material because of the improved bevel angle, i.e., a more "square" cut can be obtained. Further, water injection torches exhibit less "top edge rounding" where the corners of the cut edge closest to the torch become undesirably rounded as a result of the heat. Thus, water injection plasma arc cutting torches are dependent upon many factors for proper operation under various service demands. However, a common characteristic of conventional water injection plasma arc torches is the use of a high operational current, generally greater than 200 amperes, to maintain a sufficient plasma arc.
When cutting thin section workpieces, such as metal plates having a thickness of less than about 0.5 inches, it is desirable to use less current than with thicker workpieces. A lower current is more efficient. While it is theoretically possible to use a high current rating torch and simply reduce the current through the torch (such as using a 400 amp torch with only 200 amps of current), the resultant arc is "bushier" and less stiff, resulting in a cut which is not sufficiently uniform. In addition, the conventional view is that water injection is impractical for use with low current plasma arc torches, which generally operate at a current of 200 amperes or less, due to the energy depleting effect of the water on the plasma. The water can also deflect the arc in one direction so that the bevel angle of the cut depends in part on the direction of movement of the torch relative to the workpiece. As such, a secondary gas flow is used to cool the nozzle and constrain the arc for low current applications.
A secondary gas flow is less efficient than water in providing cooling adjacent the cut in the workpiece and, compared to water injection torches, produces a lower quality cut. One example is U.S. Pat. No. 5,132,512 to Sanders et al. which discloses a plasma arc torch capable of operating at currents of 200 amperes or less. This torch uses an insulated shield on the tip of the nozzle to guide and regulate a secondary gas flow through the nozzle. The secondary gas flow surrounds the plasma arc to provide cooling and stabilize the arc. The '512 Sanders et al. patent also notes that water injection becomes less practical in the 0-200 ampere operating current range since the water draws too much energy from the plasma. Nevertheless, there exists a need for a water injection plasma arc torch capable of operating at current levels generally less than 200 amperes.
Further, U.S. Pat. No. 5,079,403 to Sturges et al. discloses a plasma arc torch capable of operating at current levels between 45-250 amperes. This torch uses a nozzle, configured to discharge the plasma at a supersonic velocity in the form of a collimated stream, to obtain faster and squarer cutting of the workpiece. This torch is further provided with cooling water circulated within the body of the torch. However, the water is intended solely for cooling the body and nozzle of the torch and does not flow through the nozzle. The water enters and exits through the body of the torch without interacting with the plasma arc. Thus, the '403 Sturges et al. patent does not disclose a water injection plasma arc torch capable of operating at low current levels.
In contrast, U.S. Pat. No. 4,311,897 to Yerushalmy discloses a water injection plasma arc torch which uses water to intensify and collimate the arc. Stated operational currents for this torch range from 275-400 amperes. Similarly, U.S. Pat. No. 5,660,743 to Nemchinsky discloses a water injection plasma arc torch which cools the nozzle assembly and constricts the arc without unduly cooling the arc. This patent states an operational current of 350 amperes. Further, U.S. Pat. No. 4,954,688 to Winterfeldt also discloses a water injection plasma arc torch . The patent gives an operating example at a current of 400 amperes.
As such, it is known to those skilled in the art that water injection plasma arc torches are desirable for cut quality, but that the operation of water injection torches below 200 amperes has not been practical. The water flow excessively lessens the energy of the arc and can deflect the arc. Accordingly, there is a need in the art for a water injection torch which can operate efficiently and effectively at less than 200 amps without suffering a deflected or depleted arc. Such a torch would also preferably be able to produce superior cut quality on thin metal plates with a very shallow bevel angle and little top edge rounding.